Output drive circuits for power applications require rugged high-breakdown voltage transistors that can withstand a harsh high-voltage, high-current environment. A common transistor for such applications is the lateral double-diffused metal-oxide-semiconductor transistor (LDMOS), which is, despite its name, typically has a gate composed of highly-doped polycrystalline silicon rather than metal. FIG. 1 shows a typical LDMOS transistor. In operation, the source and back gate are typically coupled to electrical ground and the drain is coupled to a positive voltage supply. In the transistor's active state, a channel is formed in the p-type region beneath the poly gate by applying a positive voltage to the gate.
An important feature of power semiconductor devices is the ability to withstand high breakdown voltages when the device is in the inactive or blocking state; that is, the condition in which the drain voltage is positive and the gate voltage is less than the threshold voltage required to form a channel in the p-type region. In this condition, the transistor shown in FIG. 1 forms a depletion region at the p-n junction 10. The depletion region withstands increasingly large positive drain voltages until the avalanche breakdown condition is reached. It is desirable that the transistor have as high a breakdown voltage as possible without sacrificing other device parameters such as on-resistance and the ability to withstand electrical transients.